Quantitative aspects of ribosome assembly in HeLa cells are being investigated. Absence of polls of nearly all ribosomal proteins in the nucleolus has been demonstrated. The sole exception, protein L5, is represented by approximately 10 copies per nascent ribosome in the nucleolus. The function of L5 in ribosome assembly will be studied further. Measurements of the stabilites of each ribosomal protein have been made. It has been shown that three proteins can join the large ribosomal subunit in the cytoplasm. The approximate order of assembly of Ribosomal proteins with RNA has been determined. The lifetimes of the various ribosomal RNA precursors will be measured.